1. Field of the Invention
The present invention relates in general to a thin-film transistor, and in particular, to a thin-film transistor for the use in an LSI and a liquid crystal display (LCD).
2. Description of the Related Art
FIG. 22 is a cross section of a conventional thin-film transistor (which will be referred to as a "TFT" hereinafter) disclosed, for example, in Japanese Patent Laying-Open No. 60-136259. An insulating film 2 is provided on a substrate 1. A gate electrode 3 is provided on the insulating film 2. The gate electrode 3 is covered with a gate insulating film 4 provided on the insulating film 2. On the substrate 1, there is also provided a channel polysilicon layer 5 covering a gate electrode 3 with a gate insulating film 4 therebetween. The channel polysilicon layer 5 is divided into a source region 5a, an active layer 5b forming a channel region and a drain region 5c. The gate electrode 3, source region 5a, channel region (5b) and drain region 5c form the TFT. The channel polysilicon layer 5 is covered with a silicon oxide film 6 provided on the substrate 1. On the silicon oxide film 6, there is provided a plasma silicon nitride film 7 which has a thickness of 0.5 .mu.m to 1.0 .mu.m and is formed by a plasma chemical vapor deposition method (which will be refereed to as a "plasma CVD method"). The plasma silicon nitride film 7 is used as a protection film of the TFT, and is produced using dichlorosilane and ammonia.
Now, function of the plasma silicon nitride film will be described below. As already described, a major purpose of the plasma silicon nitride film 7 is to provide a protection film of the TFT. However, it is formed by the plasma CVD method, so that a large amount of hydrogen is contained in the film. As a result, the hydrogen passes through the silicon oxide film 6 and enters the active layer 5b of the TFT due to the anneal processing effected after the formation of film. Thereby, dangling bonds which exist in the polysilicon forming the active layer 5b are sealed with hydrogen atoms as shown in FIG. 24, resulting in reduction of the dangling bonds, i.e., in hydrogenation. As a result, the TFT can have preferable characteristics in which an off-current (condition of V.sub.d &lt;0 and V.sub.g =0) of the TFT is reduced and an on-current (condition of V.sub.g =V.sub.d &lt;0) is increased.
Then, a bias high-temperature stress test which was carried out for evaluating reliability will be described below. In the bias high-temperature test (which will be referred to simply as "BT stress"), heat and bias are applied to the gate electrode, and a load against the device is determined. More specifically, a degree of change of the characteristics relating to the drain current and the gate voltage is determined. The result is shown in FIG. 25. Referring to FIG. 25, the BT stress causes V.sub.th. (threshold voltage) to change negatively and thus the on-current decreases. FIG. 26 shows a result obtained by comparison between two kinds of TFTs, i.e., the TFT in which hydrogenation has been caused by hydrogen coming from the plasma silicon nitride film 7 and the TFT in which hydrogen has not been caused. As can be seen from FIG. 26, V.sub.th changes to a larger extent in the TFT in which hydrogenation has been caused. From this, it can be considered that the change of V.sub.th. is caused by such a mechanism that Si--H bonds in the active layer 5b of the TFT are dissociated to form an interface level as represented by formula (1) in FIG. 27, and further the hydrogen produced thereby reacts with oxygen atoms in the gate oxide film 4 as represented by formula (2), producing fixed positive charges at the interface.
In the conventional TFT, as described above, the polysilicon in the active layer, ie., channel is hydrogenated owing to provision of the plasma silicon nitride film 7, so that the TFT has good characteristics that the off-current is reduced and the on-current is increased. However, the BT stress may change the characteristics to a large extent, so that a high reliability for a long term may not be obtained.